in which we check for confusable Unicodepoints in float literal exponent

The `FatalError.raise()` might seem unmotivated (in most places in
the compiler, `err.emit()` suffices), but it's actually used to
maintain behavior (viz., stop lexing, don't emit potentially spurious
errors looking for the next token after the bad Unicodepoint in the
exponent): the previous revision's `self.err_span_` ultimately calls
`Handler::emit`, which aborts if the `Handler`'s continue_after_error
flag is set, which seems to typically be true during lexing (see
`phase_1_parse_input` and and how `CompileController::basic` has
`continue_parse_after_error: false` in librustc_driver).

Also, let's avoid apostrophes in error messages (the present author
would argue that users expect a reassuringly detached, formal,
above-it-all tone from a Serious tool like a compiler), and use an
RLS-friendly structured suggestion.

Resolves #49746.

src/test/ui/did_you_mean/issue-49746-unicode-confusable-in-float-literal-expt.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+const UNIVERSAL_GRAVITATIONAL_CONSTANT = 6.674e−11; // m³⋅kg⁻¹⋅s⁻²
+//~^ ERROR expected at least one digit in exponent
+
+fn main() {}

src/test/ui/did_you_mean/issue-49746-unicode-confusable-in-float-literal-expt.stderr

@@ -0,0 +1,12 @@
+error: expected at least one digit in exponent
+  --> $DIR/issue-49746-unicode-confusable-in-float-literal-expt.rs:11:48
+   |
+LL | const UNIVERSAL_GRAVITATIONAL_CONSTANT = 6.674e−11; // m³⋅kg⁻¹⋅s⁻²
+   |                                                ^
+help: Unicode character '−' (Minus Sign) looks like '-' (Minus/Hyphen), but it is not
+   |
+LL | const UNIVERSAL_GRAVITATIONAL_CONSTANT = 6.674e-11; // m³⋅kg⁻¹⋅s⁻²
+   |                                                ^
+
+error: aborting due to previous error
+